Chalcedony is a variety of quartz with a very important difference that sets it apart. Chalcedony is cryptocrystalline form of quartz that means no visibly seen crystals are formed, unlike quartz that is formed as crystals that can be seen easily with naked eye.
Chalcedony is often found in grayish blue color, though some other colors like yellow can be found sometimes.
Chalcedony is not a very precious gemstone nor it is a very rare stone, but it is an extremely important gemstone because some very well known and widely used gems like agate, onyx and jasper are all varieties of chalcedony.
The luster of chalcedony is like that of carnelian and both have somewhat waxy luster that is not very vitreous (glass like).